Semiconductor structures having layers formed of Group II-VI compounds or alloys are of interest in the fabrication of infrared radiation detectors and imagers. For example, CdTe, HgTe, and Hg.sub.1-x Cd.sub.x Te are narrow bandgap Group H-Group VI semiconductors which may be utilized for emitting and absorbing infrared radiation in several regions of the infrared band. Such materials are often employed in the fabrication of infrared focal plane detector arrays, in which they serve as photodetectors, and in infrared light sources such as lasers, in which they serve as emitters. Since both the photodetector and radiation emitter devices require p-n junctions, p-type doping of at least one Group II-Group VI layer is necessary in each.
In certain applications, because of the necessity for tailoring such semiconductor structures with extremely sharp transitions between layers, the layers are advantageously formed by epitaxial growth processes. Molecular beam epitaxy (MBE) is an important technique utilized in the formation of device structures requiring very thin layers, such as double-layer heterojunction photodiodes, or two-color photodiodes, in which more than two active layers are required. In such device structures, molecular beam epitaxial growth processes have been employed for forming layers of both p-doped and n-doped Group II-Group VI materials.
MBE techniques are generally known in the art and have been widely discussed in the literature. See, for example, the following articles, which are hereby incorporated by reference: A. Y. Cho and J. R. Arthur, in Progress in Solid State Chemistry, edited by J. McCaldin and G. Somorjai (Pergamon, N.Y. 1975), Vol. 10, p. 157; L. L. Chang, in Handbook on Semiconductors, edited by S. P. Keller (North-Holland, Amsterdam, 1980), Vol. 3 Chapter 9; C. E. C. Wood, in Physics of Thin Films, edited by C. Haft and M. Francombe (Academic, New York, 1980), Vol. 11, p. 35; C. T. Foxon and B. A. Joyce, in Current Topics in Materials Science, edited by E. Kaldis (North-Holland, Amsterdam, 1981), Vol. 7, Chapter 1. Articles relating to p-type doping of Group II-Group VI layers during MBE growth include J. M. Arias, et al., J. Vac. Sci. Technology A8, pp. 1025-1033 (1990), which are also incorporated by reference.
As noted, molecular beam epitaxy provides a number of advantages in the formation of various semiconductor structures. In general, MBE processes permit the growth of thin films with extremely high crystalline quality. Normally in such MBE processes, neutral molecular or atomic species are directed onto a suitable substrate in a vacuum, the substrate being heated to a temperature sufficient to permit deposited atoms to move laterally for average distances of at least several angstroms to permit the deposited atoms to find their energetically preferred sites. P-type doping of Group II-Group VI materials during epitaxial growth processes has been somewhat difficult, and it has been particularly difficult in the molecular beam epitaxial growth of mercury-containing layers, because of the high volatility of mercury. Accordingly, relatively low substrate temperatures, e.g., 185.degree. C.-200.degree. C., have been required. When it has been attempted to introduce a p-type dopant such as arsenic in association with the MBE growth of Group II-Group VI layers, particularly in the formation of p-Hg.sub.1-x Cd.sub.x Te, the MBE process has become further complicated, as will be discussed in the following section.